Abstract

A growth trial was conducted with a
total of 40 crossbred castrate males and female pigs (1:1) of approximately 35 kg initial
live weight distributed at random into five treatments to study the effect of feeding ad
libitum diets, in which maize and sorghum were substituted by graded levels of sweet
potato (Ipomoea batatas L.) root meal (SPRM, 0, 25, 50, 75 and 100%
respectively).

Animals receiving the diet with no
cereals had the poorer performance traits as compared with those from the control diet
(SPRM, 0%) whereas those fed graded levels of SPRM showed an intermediate response, during
growing (35-60 kg) and finishing (60-90 kg) phases. Overall, SPRM had no influence on feed
intake and very little effect on daily gain, but feed conversion ratio was
negatively affected by the presence of SPRM (R2 = 0.81). Results from pigs with
75% of substitution of cereals in the diet were similar to the others with lower levels of
sweet potato in the feed. There was no effect of treatment on carcass traits. An increase
in pancreas weight found in pigs fed graded levels of SPRM suggests that some
anti-nutritional factor could have negatively affected performance traits of animals
through some disturbance of digestive processes.

It is suggested that sweet potato
root meal can provide 54and 58% of the diet during the growing and the finishing phase,
respectively, making it possible to replace 75% of the cereal components.

Introduction

Sweet potato (Ipomoea batatas (Lam.) L.) is
a tropical convulvalacea which has received some attention as a possible animal feed
(Machin 1992; Scott 1992), due to its relatively abundance in the tropical area of the
World. Some literature reviews have been published concerning the use of the roots for
growing pigs (Nwokolo 1990; Pérez 1997).

Although sweet potato roots possess several
advantages to be used as a common animal feedstuff, such as its high rate of growth year
round with a minimum of culturing manipulation (González 1994; González and Díaz 1997),
there are some constraints concerning its use, and its main limitation could be that sweet
potato is a human staple food too (Wolfe 1992). However, in Venezuela, there is no marked
human preference for sweet potato root products, and perhaps this is one reason which
could explain why sweet potato is not commonly produced in this country in relatively high
amounts.

The aim of the present experiment was the
evaluation of performance and carcass traits of pigs fed sweet potato root meal as a
replacement of conventional cereal ingredients during the growing-finishing period.

Materials and Methods

A growth trial was conducted with a total of 40
crossbred castrate male and female pigs (1:1), of approximately 35 kg initial live weight
distributed at random into five treatments, to study the effect of feeds in which maize
and sorghum were substituted by graded levels of sweet potato (Ipomoea batatas L.)
root meal (SPRM, 0, 25, 50, 75 and 100% respectively). The sweet potato tubers were
harvested from a plantation of the Faculty of Agronomy at Maracay, and were from an
improved cultivar. The roots were chopped after harvest, sun-dried and ground prior to
mixing with the other ingredients of the diets. The growth trial was divided into a
growing and a finishing period of 49 and 46 days on average, respectively. Increasing
proportions of soya bean meal were added to the diets containing increasing amounts of
sweet potato root meal, to compensate for the lower protein content of the sweet potato
compared with maize and sorghum.

The protein content of the diets was determined by
a Kjeldahl procedure following AOAC (1990) recommendations. Gross energy was measured by
combusting the samples in an adiabatic bomb calorimeter.

Table 1. Characteristics
of the diets during the growing period (35-60 kg)

Substitution of cereals by sweet potato root meal, %

0

25

50

75

100

Ingredients

Soybean oil
meal

17.80

19.90

22.70

24.50

26.00

Maize meal

38.30

28.40

18.30

9.00

-

Sorghum meal

38.30

28.40

18.30

9.00

-

Sweet potato
root meal

-

18.90

36.60

54.20

71.70

Tallow

1.50

1.00

1.00

0.50

-

CaPO4H, 2H2O

3.30

0.40

0.25

0.10

-

CaCO3

-

1.60

1.60

1.60

1.60

DL-methionine

0.10

0.70

0.55

0.40

-

NaCl

0.50

0.50

0.50

0.50

0.50

Vitamin
premix1

0.15

0.15

0.15

0.15

0.15

Mineral
premix1

0.05

0.05

0.05

0.05

0.05

Analysis

Nx6.25, % DM

16.23

15.94

15.79

15.70

15.50

Gross energy.
KJ/g DM

17.91

17.64

17.36

17.27

16.61

*
According to NRC (1998) recommendations

Table 2 lists the composition of the diets used
during the finishing period. The characteristics of the control diet were similar to that
of the growing period, and again, a decrease in protein and gross energy content was
apparent with increasing levels of SPRM in the diet.

Table 2. Characteristics
of the diets during the finishing period (60-90 kg)

Substitution of cereals by sweet potato root meal, %

0

25

50

75

100

Ingredients

Soybean oil
meal

17.14

14.89

17.49

19.70

21.40

Maize meal

40.06

30.25

19.52

9.63

-

Sorghum meal

40.06

30.25

19.52

9.63

-

Sweet potato
root meal

-

20.20

39.00

57.70

76.30

Tallow

-

1.00

1.00

0.50

-

CaPO4H.2H2O

2.04

2.21

2.47

1.24

-

CaCO3

-

0.50

0.30

0.90

1.60

NaCl

0.50

0.50

0.50

0.50

0.50

Vitamins
premix1

0.15

0.15

0.15

0.15

0.15

Mineral
premix1

0.05

0.05

0.05

0.05

0.05

Analysis

Nx6.25, % DM

15.52

14.98

14.22

13.48

13.65

Gross energy,
KJ/g DM

17.82

17.81

17.02

17.22

17.56

1
According to NRC (1998) recommendations

The animals were housed in an open stable with
cement floor provided with an individual trough and drinking nipple. Two animals, one
castrate male and one female, were housed per pen of 1.5x2.5 m.

When the animals
reached 90 kg liveweight they were fasted during 24 hours, then slaughtered for carcass
analysis. Carcass yield was calculated and after refrigeration at 4oC during 24 hours,
different carcass traits were determined in the left half of the cold material. In
addition, fresh liver and pancreas weight were determined at sacrifice of the
animals.

The data were analyzed
according to a one-way classification and mean comparison was made by the Duncans
new multiple range and multiple F test (Duncan 1955) as described by Steel and Torrie (1980). The pen with two animals was considered the experimental unit.

Results

A slight difference in initial live weight was
obtained in this trial (Table 3), but this effect was not significant, and therefore, no
adjustment of data was made in any performance trait, during the conduction of the
analysis of variance. Significant differences (P<0.05) among treatments were found for
final weight of the animals. In this connection, it was found that treatments containing
SPRM from 0 to 50% of cereal substitution did not have difference in growth rate. However,
75 and 100% of replacement of maize and sorghum determined a significantly (P<0.05) low
daily gain in the pigs as compared to that of the animals from the other treatments. There
was no treatment effect in feed consumption, and feed conversion was the lowest
(P<0.05) in pigs fed SPRM with no cereals in the diet.

* P<0.05
abc Means without letter in common in the same row differ significantly (P<0.05)

The same trend
observed during the growing period for daily gain and feed conversion ratio was found
during the finishing period (Table 4), with significantly poorest values (P<0.05) for
animals fed the highest substitution of maize and sorghum by SPRM in the diet. No
treatment effect was observed on voluntary feed intake.

* P<0.05
abc Means without letter in common in the same row differ significantly (P<0.05)

Overall performance traits for the growing and
finishing periods are listed in Table 5. Obviously, these indices reflected the dietary
influence found in both periods, and therefore, although differences in the increase in
daily weight was only evident (P<0.05) for the pigs fed no cereals in the diet as
compared with the other treatments, a decrease in daily gain was more evident with the
increase of graded levels of SPRM in the diet (P<0.05). Nevertheless, treatments with
SPRM substituting up to 50% of the maize and sorghum were similar to the control diet.
Since the trial was finished when the animals reached approximately 90 kg of live weight,
days in test were significantly higher in pigs fed diets with 100% of substitution of
cereals by SPRM (P<0.05).

The analysis of regression revealed that treatment
influence was not evident in feed intake and very little effect was found of SPRM on mean
daily gain (P<0.115). However, feed conversion ratio showed a strong, negative response
to the introduction of SPRM in the diets of the pigs, with poorer values for animals fed
diets with 75 and 100% of substitution of cereals by SPRM with a straight relationship (R2
= 0.806; P<0.001) between both variables (Figure 1).

Figure 1. Effect
of sweet potato root meal on feed

Data corresponding to carcass traits are set out
in Table 6. Not significant differences were encountered in any of the measurement made in
the animal's carcass as influenced by the level of dietary SPRM. On the other hand, SPRM
in the diet did not influence the weight of the liver, but the pancreas showed a
significant dietary influence, with significantly (P<0.05) heaviest organs in pigs fed
least cereals in the diet.

Table 6. Carcass characteristics in pigs fed sweet potato root meal1

Cereal
substitution by sweet potato root meal, %

0

25

50

75

100

SE ±

Carcass traits

Yield, %

67.0

66.2

66.3

65.5

66.7

2.4

Half carcass, kg

31.2

30.5

30.2

29.6

28.9

1.1

Backfat thickness, cm

First rib

3.89

4.41

4.53

4.93

4.57

0.84

Last rib

2.56

2.51

2.68

2.63

2.45

0.43

Last lumbar vertebra

2.20

2.46

2.34

2.47

2.29

0.50

Mean

2.89

3.13

3.14

3.34

3.10

0.49

Longissimus area, cm2

24.60

26.62

24.12

25.47

24.49

4.41

Ham with fat, kg

7.87

7.81

7.35

7.36

7.18

0.41

Loin with fat, kg

8.91

8.79

8.07

9.10

8.82

0.41

Organs

Liver, kg

1.31

1.21

1.42

1.31

1.41

0.15

Pancreas, g

105a

93a

118ab

137bc

153c

35*

1 Initial live weight, 35 kg
* P<0.05
abc Means without letter in common in the same row differ significantly (P<0.05)

Discussion

Results from the current investigation concerning
feed consumption are not in agreement with those from other studies (Alvarado et al 1977;
Acurero et al 1981; Moita et al 1991) where it has been observed that growing pigs
decreased its voluntary feed intake with increasing levels of SPRM in the diet.

Other investigations have obtained a similar
response in growth rate to that found in this investigation, since several results have
indicated a depression in daily gain when pigs are allowed to eat diets with sweet potato
roots as the only source of energy (Koo and Kim 1974; Lee and Lee 1979; Soares 1989; Moita
et al 1991). In this connection, previous data from our laboratory indicated that when
roots and foliage from sweet potato were given mixed to the pigs, only a depression in
growth rate was observed when no cereals were used in the diet (González et al 1997;
García 1998).

Data from the present trial are in accordance with
other showing the same trend for feed conversion ratio (Alvarado et al 1977; Moita et al
1991). However, there are several reports suggesting that the efficiency of feed
conversion is not deteriorated when sweet potato roots are the only source of energy in
the diet (Tai and Lei 1970; Lee 1975; Lee and Lee 1979; Lee 1980; Lee and Yang
1982).

Little is known concerning the influence of the
inclusion of sweet potato roots in the diet on several carcass traits of pigs. However,
some data refer no influence (Lee 1975) or even an improvement effect (Lee and Lee 1979)
of sweet potato roots on carcass traits of pigs. The current study did not provide a
comprehensive explanation for the increase of pancreas weight as affected by increased
levels of sweet potato roots in the diet. However, since sweet potato roots do contain
some anti-nutritional factors with anti-trypsin activity (see Pérez 1997), and this
compounds could be influencing digestive processes where the pancreas is decisive. Ly et
al (1999) found that in vitro pepsin/pancreatin digestibility of nutrients from sweet
potato tubers is rather low, thus supporting results from pancreas enlargement observed in
the present investigation. A deficiency in pre-caecal digestibility of nutrients in pigs
fed sweet potato roots must be directly related with a deterioration in performance traits
such as that found in the current study.

Conclusions

It is suggested that with 54.2 and 57.7% of sweet
potato root meal in the diet during the growing and the finishing phase, it is possible
the substitution of 75% of the cereal components of the diets for pigs. More investigation
is needed to improve the use of high level of sweet potato roots in diets for pigs.

Acknowledgments

The authors thank the assistance of the librarian
from the Swine Research Institute at Havana for his assistance in literature search, and
to the staff from the Pig's Unit at the Faculty of Agronomy at Maracay for his support
during the conduction of the present experiment.